This invention relates to an ink jet recording head that records dots on a recording medium by splashing ink in the form of droplets, utilizing mechanical energy produced by a piezoelectric element.
One typical example of an ink jet head that makes a recording by jetting ink is, as disclosed in U.S. Pat. No. 3,946,398, constructed so that the ink is attracted by changing the volume of an ink chamber while vibrating a piezoelectric element at a first timing, and splashed onto a recording sheet in the form of droplets by applying a pressure at a second timing. As proposed in Japanese Patent Unexamined Publication No. 161935/1979, another example has such a construction that a heating element is contained in an extremely small nozzle-forming member; that bubbles are instantaneously produced in such nozzle-forming member by heat energy derived from the heating element; and that ink is jetted out by the expanding force of bubbles.
The recording heads utilizing the principle of a pump produce the minimum size of an ink droplet which is in the order of 100 to 200 .mu.m, thereby implementing a recording density of 150 to 300 DPI. Although this can ensure practically applicable high-quality recording of binary data such as character data, data including different levels of dot data such as photographs and pictures cannot be reproduced on a dot basis. For this reason, it is required that a single pixel, which is a unit of image data, consist of a plurality of dots and that a density level be expressed by increasing or decreasing the number of dots. And this further requires that an area for a plurality of dots be provided to print a single pixel and that the document image data be sampled at an appropriate density to form the print data. Thus, the number of pixels in the printed document becomes smaller than that in the document image, thereby reducing the resolution.
To overcome this problem, it has been proposed to provide a recording head such that a supersonic vibrating element having an acoustic lens is immersed in ink and that ink droplets are jetted by a supersonic beam converged at a single point (Japanese Patent Unexamined Publication No. 166547/1988). According to this concept, the amount of ink to be applied to a recording sheet is proportional to the operating time of the supersonic vibrating element. Thus, the optical density of a dot can be adjusted freely by controlling the operating time and this allows image data to be recorded without reducing their recording density.
However, since the vibrational energy utilization efficiency is low in such a system, it requires a large supersonic vibrating element, which not only makes the recording head larger in structure, but also imposes a restriction in the installing direction of the recording head from the requirement that the surface of the ink be exposed.
To overcome these problems, the applicant of the invention has provided a nozzleless ink jet recording head as disclosed in U.S. Ser. No. 492,446, U.S. Pat. No. 5,063,396, issued Nov. 5, 1991. This nozzleless ink jet recording head includes: a propagating body that has a propagating surface for propagating a surface elastic wave to an edge portion that is supplied with the ink; and means for producing the surface elastic wave to this propagating body. This head can provide a sufficient splashing force irrespective of the size of ink particles, thereby not only controlling the size of a printable dot, but also contributing to simplifying the structure and improving the durability by eliminating the ink pressure chamber. But at the same time, a propagating path suitable for producing a surface wave is required since the head utilizes the surface elastic wave, and this demands not only a vibrating substrate of a proper size, but also a high level of drive frequency and complicated signal processing circuits.